The present invention relates to a viscous fluid coupling device, and in particular to a viscous fluid coupling device which is used to establish a connection between an output shaft of a vehicle engine and a cooling fan assembly.
A conventional viscous fluid coupling device of the kind is disclosed in Japanese Patent Publication No.Sho59(1984)-28778 which was issued on Jul. 16, 1984. This conventional device has an input member to be driven by a vehicle engine, an output member rotatably mounted on the input member via bearing, a fan assembly connected to the output member, a rotor with a passage secured to the input member and accommodated in a space defined between the input member and the output member, and a labyrinth mechanism provided on an outer periphery of the rotor. The space is divided into an operating chamber and a reservoir chamber by a partition plate with a hole. The partition plate is provided with a thermally responsive or first valve in order that the hole can establish a fluid communication between the reservoir chamber and the operating chamber while an environment temperature is above a set value. A second valve is provided with on the rotor which is set to close the passage when the rotation number of the input member exceeds a set value. An amount of viscous fluid in the space is set to be circulated through the reservoir chamber, the hole, the passage, and the labyrinth mechanism. The rotation number of the output member is set to depend on the quantity of the viscous fluid to be supplied to the labyrinth mechanism which is related to the conditions of the first valve and the second valve.
In case that the foregoing viscous fluid coupling device is in operation under a hot environment, the first valve is being opened, when the centrifugal force exceeds the set value as a result of a high speed rotation of the input member, the second valve is closed, which results in that the increase of the rotation number of the output member is restricted despite any further increase of the rotation number of the input member.
However, when the second valve is brought into opened condition again when the rotation number of the input member becomes less than the set value concurrent with the lowering in the temperature, both of the first valve and the second valve are fully opened, resulting in that the large amount of viscous fluid is supplied to the the labyrinth mechanism. Thus, the rotational number of the output member is increased despite of the cold environment under which the high rotation of the fan assembly by the output member is not required.